The classic ‘gold standard’ for measuring ion concentration is a glass electrode. A glass electrode system is a measurement device having a type of ion-selective, or ion-sensitive, electrode made of a doped glass membrane that is sensitive to a specific ion. FIG. 1 illustrates generally a block diagram of a glass electrode system 100. The glass electrode system 100 can include two electrodes, one for sensing the target ion 102, such as hydrogen ions (H+), and one for providing a reference 101. In the example of FIG. 1, the two electrodes 101, 102 are located in liquids inside the glass electrode probe 103. Both electrodes 101, 102 provide a voltage at the interface to the liquid under test, the reference electrode 101 provides a nearly constant voltage which can be independent of the target ion concentration, and the sensing or indicating electrode 102 provides a voltage that varies with the concentration of the target ion. The voltages are conveyed via a wire medium 105 to an amplifier system 104 and then processed for display or data collection. In certain situations the voltages can be converted to digital values, via an analog-to digital converter (ADC) 111, for example, and further processed digitally. Characteristics of glass electrode systems that can interfere with measurements include output impedance on the order of 20 mega ohms (MΩ) to 800 MΩ, noise coupling to the wire medium 105 and expense of the wire medium and the probe 103. In addition to glass electrodes, there are low performance disposable solutions know as Litmus papers which are passive strips which change color in response to the concentration of a target ion. The litmus paper type measurements are not considered precision because, for example, using litmus papers is subject to a subjective reading of color, the papers themselves can interfere with small samples and it can be difficult to fabricate papers which cover full ion concentration ranges.
A relatively new solid state technology known as ion-sensitive, field effect transistor (ISFET) has emerged that is based on an activated FET gate, which can produce an output source-drain current related to the exponential of pH (Non-Nernstian). The technology has been used in DNA sequencing and food process control, but can have issues with charge trapping, hysteresis and drift that may have limited the technologies more widespread use. In addition, an ISFET provides an indicting electrode only, and an additional external reference electrode is usually required.